1. Field of the Invention
The present invention relates to a light source apparatus, an electronic device, and a control method therefor.
2. Description of the Related Art
A power factor correction (PFC) circuit has been used for suppressing harmonic current in display devices requiring high-capacity power sources. In the case of a PFC circuit constituted by a boost-type converter, the output voltage Vout of the PFC circuit should be set to a value higher than the peak voltage Vinpeak of the input voltage Vin of the PFC circuit (see FIG. 8A).
This is because when the output voltage Vout of the PFC circuit is equal to the input voltage Vin of the PFC circuit, the PFC circuit stops and the harmonic current cannot be suppressed. The peak voltage Vinpeak of the input voltage Vin of the PFC circuit, as referred to herein, is a peak value (about 374 V) of the pulsating voltage after the maximum value (for example, AC 264 V) of the voltage supplied form the commercial power source has been rectified.
Meanwhile, in order to reduce the rated voltage of the output capacitor used in the PFC circuit or reduce the difference in electric potential between the primary and secondary sides, it is preferred that a lower output voltage Vout of the PFC circuit be obtained.
However, when the load current of the PFC circuit changes abruptly due to increase in brightness of a backlight (light source) installed in a display device, load fluctuations occur and the output voltage Vout of the PFC circuit decreases. Therefore, it is necessary that the output voltage Vout be set to a value sufficiently higher than the peak voltage Vinpeak of the input voltage Vin of the PFC circuit (see FIG. 8B).
A method of dividing a backlight into a plurality of regions and shifting the ON/OFF timing for each region has been suggested as a method for suppressing the decrease in the output voltage Vout of the PFC circuit (see, for example, Japanese Patent Application Publication No. 2002-43089).
Further, taking into account that the backlight brightness is changed with a constant period, a method can be considered by which backlight brightness variations are synchronized with the commercial power supply frequency and the backlight brightness is prevented from changing in the vicinity of the peak voltage Vinpeak of the input voltage Vin of the PFC circuit. However, such synchronization is actually difficult to achieve because of a spread in the commercial power supply frequency and also because the commercial power supply frequency varies depending on a region (50 Hz/60 Hz).
However, with the above-described conventional technique, a high output voltage Vout should be set with consideration for the decrease in the output voltage Vout of the PFC circuit following the increase in backlight brightness. Therefore, a large-size output capacitor with a high rated voltage is necessary for the PFC circuit and the power source for a display device is difficult to miniaturize. Further, since the difference in voltage between the primary and secondary sides is increased when the output voltage Vout of the PFC circuit increases, a large insulation distance should be ensured and the power source for a display device is difficult to miniaturize.